If a volunteer cant login then follow the checks below:
Are they using the correct email address?
You can check this by going to [ADMIN MENU] VOLUNTEERS MANAGEMENT > FIND VOLUNTEERS
Then search the the volunteer and click on their name, this will take you to their volunteer profile page.
From here you can see the email address that the volunteer used to register.
Get them to reset their password
If they are using the correct email address then they can reset the password via their login screen, they just need to click on the FORGOTTEN YOUR LOGIN DETAILS? link beneath the password box.
They can type in their email address and it will send them an email requesting them to reset their password.
PS: Admins can also reset the volunteers password via the volunteer profile page, but this is not recommended as volunteers will often want to use a password that is used for other accounts they hold.